The present invention relates to a method of and a mechanism for feeding a sheet, and more particularly to a method of and a mechanism for feeding a sheet such as a stimulable phosphor sheet, a film, or the like with image information recorded thereon by attracting the sheet, the film, or the like to a feed belt with at least two vacuum chambers to smoothly move the sheet, the film, or the like in an auxiliary scanning direction while the sheet, the film, or the like is being scanned by a one-dimensionally deflected light beam in a main scanning direction substantially normal to the auxiliary scanning direction, so that the image information can accurately be read and/or recorded.
There has recently been known a radiation image information recording and reproducing system for producing the radiation-transmitted image of an object using a stimulable phosphor material capable of emitting light upon exposure to stimulating rays When a certain phosphor is exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, the phosphor stores a part of the energy of the radiation. When the phosphor exposed to the radiation is subsequently exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy of the radiation. The phosphor exhibiting such a property is referred to as a "stimulable phosphor".
In the radiation image recording and reproducing system employing such a stimulable phosphor, the radiation image information of an object such as a human body is stored in a sheet having a layer of stimulable phosphor, and then the stimulable phosphor sheet is scanned with stimulating rays such as a laser beam to cause the stimulable phosphor sheet to emit light representative of the radiation image. The emitted light is then photoelectrically detected to produce an image information signal that is electrically processed for generating image information which is recorded as a visible image on a recording medium such as a photosensitive material or displayed as a visible image on a CRT or the like.
The radiation image recorded on the stimulable phosphor sheet is read in the radiation image recording and reproducing system as follows:
The radiation image recording and reproducing system includes an image reader for two-dimensionally scanning the stimulable phosphor sheet with a light beam such as a laser beam to cause the stimulable phosphor sheet to emit light, and detecting the light in time series with a light detector such as a photomultiplier to obtain image information. The stimulable phosphor sheet is usually two-dimensionally scanned by deflecting the light beam and applying the deflected light beam to the stimulable phosphor sheet in a main scanning direction, and simultaneously feeding the stimulable phosphor sheet mechanically on a conveyor belt or the like in an auxiliary scanning direction substantially normal to the main scanning direction.
The image information thus retrieved is then sent to an image reproducer. In the image reproducer, a laser beam which has been modulated by the image information retrieved from the stimulable phosphor sheet is applied to a photographic photosensitive member such as a photosensitive film to reproduce the image on the film. When the modulated laser beam is applied to the film, the film is mechanically fed in an auxiliary scanning direction by a feed belt.
Where the stimulable phosphor sheet is to be fed in the auxiliary scanning direction by the feed belt, then it is necessary that the stimulable phosphor sheet be fed at a constant speed and kept flatwise. If the stimulable phosphor sheet were not fed at a constant speed, then the image recorded on the stimulable phosphor sheet would not accurately be read. If a portion of the stimulable phosphor sheet were spaced from the feed belt and no longer kept flatwise, the recorded image would be read as a distorted image. Therefore, it would be impossible to accurately read the image information borne by the stimulable phosphor sheet.
One conventional arrangement for feeding a stimulable phosphor sheet in an auxiliary scanning direction at a constant speed and flatwise employs a suction box connected to a feed belt and a vacuum suction source, The feed belt is an endless belt having a plurality of holes with the suction box disposed in the feed belt. When the vacuum suction source is operated, the suction box draws the stimulable phosphor sheet to the feed belt under a vacuum to keep the stimulable phosphor sheet flat and feed the same without displacement from the feed belt.
The suction box generally has a plurality of suction chambers arrayed in the direction in which the stimulable phosphor sheet is fed, to prevent air from leaking from the entire suction box. When the stimulable phosphor sheet is fed, the suction chambers are successively evacuated one at a time as the stimulable phosphor sheet is moved along. Therefore, the stimulable phosphor sheet is held intimately against the feed belt by at least one of the suction chambers at all times.
When the suction chambers are successively evacuated, however, the stimulable phosphor sheet may be fed at an irregular speed upon switching from one suction chamber to another suction chamber. For example, when the leading end of the stimulable phosphor sheet drawn by the first suction chamber and fed by the feed belt reaches the second suction chamber, the second suction chamber is evacuated to attract the stimulable phosphor sheet. At this time, the load imposed on the stimulable phosphor sheet as it is fed along is abruptly increased, thereby varying the interval by which the stimulable phosphor sheet is fed. When the first suction chamber is disabled after the stimulable phosphor sheet is attracted by the second suction chamber, the load on the stimulable phosphor sheet as it is fed is abruptly reduced, also varying the interval by which the stimulable phosphor sheet is fed. If the stimulable phosphor sheet is subjected to such fluctuations in the feed rate or interval, then the image information recorded on the stimulable phosphor sheet would not be read precisely, and could no longer be used for medical diagnosis or other applications which demand the high-fidelity reproduction of the recorded image.
The applicant has proposed a small-size image reading and reproducing system comprising an image reader and an image reproducer which share a common scanning system for effecting both an image reading process and an image reproducing process (see Japanese Patent Application No. 62-265012, for example). In the proposed image reading and reproducing system, an image is reproduced on a photosensitive film while the latter is being attracted by a suction box and fed by a feed belt in the same manner as when a recorded image is read from a stimulable phosphor sheet. However, a special problem arises out of the fact that the film used is considerably thinner than the stimulable phosphor sheet. When the film is attracted by first and second adjacent suction chambers of the suction box, a portion of the film on the partition between the first and second suction chambers is not attracted by the suction box. Therefore, the film has alternate attracted and unattracted portions, which make the thin film wavy in shape on the feed belt. As a consequence, the desired degree of flatness of the entire film can no longer be ensured When this happens, scanning lines on the film tend to be distorted, thus failing to reproduce the image highly accurately.